Noise suppression device



D. R. DE BOISBLANC NOISE SUPPRESSION DEVICE Filed March 7, 1949 3 Sheets-Sheet l RESISTANCE ii 0U UT E: 20 r24 7 OUTPUT F IG. 2.

INVENTOR.

D.R.IDE BOISBLANC A T TORNE rs Dec. 1953 D. R. DE BOISBLANC v 63,

NOISE SUPPRESSION DEVICE Filed March 7, 1949 :s Sheets-Sheet 2 NON LINEAR RE|STANCE NON LINEAR RESISTANCE INPUT NON LINEAR RESISTANCE I00 96 |0| 03 I06 I OUTPUT I04 97 7 NON LINEAR --rTO RECTIFIER RESISTANCE I09 CIRCUIT VOLTAGE- INVENTOR.

RESISTANCE I D R. DE BOISBLANC Mdwf FIG. 6. I

A T TORNEKS' Dec. 22, 1953 D D BOISBLANC 2,663,765

NOISE SUPPRESSION DEVICE Filed March 7, 1949 3 Sheets-Sheet 3 46 45 r 47 E 3 4| 40 j 42 n. 2

FREQUENCY '56 Knnnnnnnn UUUUUUUUUU FIG. 6.

INVENTOR. D R. DE BOI SBLANC A 7' TORNE V5 type and their resistance varies non-linearly in accordance with the voltage impressed thereacross. That is, the relation between the current and impressed voltage across one of the Thyrite resistances is not expressed by Ohms law, but rather by the relationship I :Ke", where I represents current, n is a constant greater than unity, and e represents voltage.

The voltage-resistance relationship is shown by the graph, Figure 6, in which the abscissa represents resistance and the ordinate represents voltage. From this graph, it will be noted that the resistance of each of the Thyrites i9, 29 increases in accordance with a power function as the voltage decreases. The graph shows a typical voltage-resistance characteristic but it will be understood that the curve may be made steeper or flatter by suitable choice of the kind and arrangement of material in the non-linear resistances. The response of such resistances to voltage changes is practically instantaneous and there is no time lag or averaging sheet as is noted with non-linear resistances of the hot wire type. This latter type of resistance, however, may be used in some cases with resultant elimination of the in- H tegrating circuit of the present invention.

The noise suppression circuit also includes an impedance 2| shunted across non-linear resistance 20, this impedance lay-passing a portion of the signal voltage appearing across resistance and the amount of voltage thus by-passed is dependent upon the frequency of the incoming signal. It will be noted that the resistances as, 28 and impedance 2! constitute a filter, the pass band of which is determined by the ohmic value of resistances l9 and st, for a given value of impedance 2!.

The integrating circuit l1 includes a potentiometer 22, the fixed terminals of which are shunted by a condenser 23, and the arm of which is connected to an output terminal 24, the other output terminal being connected to the junction between non-linear resistances l9 and 28. One terminal of potentiometer 22 is grounded,

and the other terminal is connected by a lead to the adjacent end of non-linear resistance 28. The integrating circuit ii is supplied with amplified, rectified signal voltage from the input terminals 12 and is. To this end, input terminal I2 is connected by a lead 21 and a coupling condenser 28 to the control grid of an amplifier tube 29. This control grid is also connected to ground by a grid resistor 30 which is shunted by a condenser 31 while the cathode of the tube is connected to ground through a biasing resistor 32 which is shunted by a by-pass condenser 33. The anode of tube 29 is connected to a positive power supply terminal 3d through a resistance 35, and it is further connected to an input lead of integrating circuit i'i through a coupling condenser 35 and a rectifier 31, the junction between the latter units being grounded through a resister 38.

The operation of the described circuit may be better understood by reference to Figure 2 which shows only the noise suppression circuit. An input signal impressed across the resistances I9, 20 produces an output voltage across the unit 28, 2! which is decreased in amplitude in accordance with the ratio between the parallel complex impedance of units 28 and 2| and the total complex impedance of the circuit. In this connection, it will be noted that the circuit functions as a voltage divider, resistance l9 forming one section of the divider, and the units 26, 2! forming the other section. For example, assuming resistances is, 29 to be of equal value, and assuming a signal frequency such that the impedance 2i is very large compared to the ohmic value of resistance as, it will be apparent that the amplitude of the output signal is 50 per cent of the amplitude of the input signal.

At a second signal frequency, where the impedance 2| is equal to the ohmic value of resistance 26, the impedance of unit 2i}, 2! is lowered, and the output signal amplitude is less than 5 per cent of the input signal amplitude.

At a third signal frequency one octave removed from the second signal frequency, the impedance 2! is equal to one-half the ohmic value of resistance 28, thereby producing an additional drop in the output signal strength.

Thus, within the range between the first and second frequencies, no detectable attenuation of the signal is produced, while beyond the second or cut-off frequency, there is an attenuation which approaches the ultimate slope of six decibels per octave.

Where attenuation of unwanted high frequency signals is desired, the impedance 2! is a condenser while, when attenuation of low frequency signals is desired, the impedance 2| is an inductance. A typical output amplitudefrequency relationship for high frequency attenuation is shown by the plot 16, Figure 7, from which it will be noted that attenuation of the signal from its initial or input amplitude H is very small (less than three decibels), until cut-01f frequency s2 is reached, after which the signal is attenuated about six decibels per octave as the frequency is increased.

The plot 48 represents the frequency-amplitude relationship when the signal amplitude is very low and, consequently, the ohmic value of resistances i9, 23 is correspondingly high. As a result, the cut-on point 42 where the reactance of condenser 21, which decreases with increasing frequency, equals one-half the relatively high ohmic value of resistance 2:; at a low frequency. The values of the circuit components are such that all high frequency signals which are inaudible to the car at such low amplitude levels are attenuated and eliminated. The signals thus eliminated would be inaudible even in the absence of noise. But the high level scratch noise which would be above the threshold level is attenuated, thus improving the apparent signal to' noise ratio.

When the amplitude of the input signal is increased, the voltage across resistances i9, 28'

is increased, and their resistance is decreased, in accordance with Figure 6. cut-01f point t? at which the reactance of condenser 25 equals half the lower ohmic value of resistance 26 occurs at a higher frequency. This is shown by the plot as, Figure 7, in which the original high signal amplitude it decreases little with frequency until the cut-off point d? is reached, this cut point being at a substantially higher frequency than the out-elf point 32, of the low amplitude curve 30. pedance 2i is a condenser, the cut-oi? frequency increases as the signal As a result, the

Thus, where iinamplitude becomes greater, and the cut-off frequency decreases as controlled by the ohmic value of the resistances is, a which, turn, varies inaccordanee the amplitude of the incoming signals. The rate at which the out-oil frequency increases with amplitude is dependent upon the voltage-resistance characteristics of the non l'irrear resistances i9, 26. If the resistance of these units decreases rapidly with increasing signal or applied voltage, the cut-off frequency increases rapidly with signal amplitude. Likewise, if the resistance of units 19, 2'9 decreases slowly with increasing signal or applied voltage, the cut-oh frequency increases slowly as the signal level increases.

It may be considered that the resistances I9, and condenser 2i constitute a low pass filter, the cut-cs frequency or "which is adjustable, in response to changes in signal amplitude, by the resultant variations in "resistance of the l8 and 2 0.

Where it is desired that unwanted low fire quency signals 'be removed from the output of the reproducer, rather than unwanted high frequency signals, the impedance 2i is an inductance, rather than a condenser. In this-case, the impedance of the inductance is high "at normal signal frequencies but decreases with decreasing frequency. The inductance of the unit 2'] is so adjusted that its impedance is equal to that of resistance :23 at a predetermined cut-off frequency, and for a preselected signal amplitude. Accordingly, at the cutoif frequency, the signal attenuation is three decibelsnvhich is the smallest volume change detectable by the human car while, at higher frequencies, the attenuation is less than three decibelsan'd is not det'ectable'hy the human ear. At frequencies below the "cutoff frequency, the signal is attenuated three decibels per octave due to the progressively decreasing impedance of the inductance, with respect to the ohmic value'offr'esistors Zlland 21.

When the amplitude of the input signal is increased, the voltage across resistances I9, 20 is increased, and their resistance 'is decreased, in accordance with Figured. Asa result, the cutoff frequency at which the im edance of inductance 2| equals the lower ohmic value of resistance 28 is decreased. Thus, where impedance "2'! is an inductance, the cut-off frequency decreases as the signal amplitude becomes greater, and the cut-on frequency increases as the amplitude is reduced. In this manner, unwanted low frequency, low amplitude noise signals are eliminated from the output of the reproducer. Here again, the rate at which the 'cutfoff frequency decreases with amplitude is dependent iipon the voltage-resistance characteristics of the nonlinear resistances I9, 20 and'the elements l9, 29, 2! in this case constitute a high pass filter, the cut-off frequency of which decreases with increasing amplitude, due to the voltage-resistance relationship established by resistances I9 and 20. Broadly speaking, therefore, it will 'be apparent that a voltage representative of signal amplitude impressed across the resistances l9 and 20 varies the impedance thereof to shift the preselected frequency range "within which signals appearing across the unit 20, 2i are attenuated by the impedance 2 I. That is, the signal appearing across the unit 20, 2! is attenuated by a decrease in the reactance ofthe impedance 2| at all frequencies within a preselected frequency range, and the impressed voltage across the "resistances 1'9, '20

varies the ohmic value thereof to 's'liiftsaid'preselected frequency range within the frequency spectrum inaccordance withthe signal amplitude.

6 Hit Will be apparent that the circuit 3 9, "w i of Figure 1 operates in the manner described in connectionwith Figure 2. Signal voltage is impressed across the resistances i 9, 20 by their mc'lusion the cathode circuit of tube, i=0, and

this voltage appears across the resistance ampedancefunit 20, 2i and across the output terminals 2'4, 25 which are effectively connected across this unit through 'a portion of potentiometer 22-. A direct voltage representative of -averag-"e signal amplitude is applied across the integrating circuit 22,23 to change the bias on tube H). bias voltage, as stated, controls the location of the attenuation range th'e lire'q'u'e'iric'y spectrum by varying the current flow through non=linear resistances f9 and 20 and tube 11). The constant of the integrating circuit, "of con se,

determines the rapidity with which the cut-' 6 frequency shifts in response to variations amplitude of the incoming signal. The potentiom- =eter 2-2 forms a br idge circuit with resistance 20 and the output load to ha'lance out voltage surges resulting from sudden changes signal amplithe noise suppressor circuit cuts out high frequency "signals which are beyond the limits of audibili'ty of the human car at *each particular amplitude "level "of the incoming signals and, when impedance "2] is an inductance, the "same effect occurs with respect to low frequency sign'als. 'In the majority of cases, use of a high frequency "filter is 'sufiicient to bring about a "sub-- stantia'l -decrease in the signal to noise ratio and, in fact, to suppress the noise to such an extent that it is not noticeable in the output of the reproducer. However, it will be apparent that two similar circuits may he used in a single repro ducer, if desired, one to suppress unwanted high frequency components and the other to suppress unwanted low frequency components.

modified noise suppression circuit is shown in 'Fig'ure'3. In this circuit, the amplifier, rectifier,-andintegrating circuit of Figure l are eliminated so that the impedance 'of the non-linear resistances is controlled by the instantaneous signal "voltage rather than by 'an average or integrated rectified signal voltage, as in Figure "1. To this end, the integrating circuit 11 is eliminated and the series 'conneotednon-"linea'r resistances are=connected directly between the cathode of tube l O'and a ground terminal 50. The output voltage appears across terminals SI and "52 "which are "connected across the resistance-impedance units 20 and 2!. The operation of this circuit is Similar 'to "that of Figure 1, except that the ohmic valueoi the resistances 19,76 is controlled by theinstantaneous signal voltage rather than by the direct voltage produced by an integrating circuit, such as circuit 22, 23 of Figure 1.

The distortion encountered with this type of circuit is very small since a cathode follower arrangement'is used so that the voltage appearing b'e'tween the cathode of tube "I'll andlground is'always'the same as the voltage impressed between the control grid of tube I and ground. As a result, neglecting the impedance 2 i, the output voltage is always one-half the input voltage if the resistances i9, 20 are of equal ohmic value or, alternatively, it bears a fixed ratio to the input voltage if these resistances are not of equal ohmic value. The only type of distortion encountered with the present circuit is that illustrated by Figure 8 wherein, for purposes of example, an input signal is shown consisting of a low frequency component 55 and a high frequency component as which are combined to form a resultant signal 5?. The described circuit is of the high frequency cut-off type and, accordingly, high frequency component 58 is eliminated from the reproducer output until the amplitude of the signal is equal to that represented by the lines 58, at which point the cut-off frequency is sufficiently high, due to increasing signal amplitude, as to admit the component 53. Accordingly, between the lines 58, the signal consists of the wave 55 by itself,-whereas, above and below the respective lines 58, the signal consists of the combined wave iorm represented by the sum of components 55 and 56. This distortion of the wave form produces no appreciable distortion of the incoming programs, however, and it may be substantially eliminated by operating the noise suppression circuit at an amplifier stage where the signal level is very high so that the amplitude represented by the distance between the lines 553 is only a small fraction of the total signal amplitude.

The distortion of the signal just described, which results from intermodulation of low frequency and high frequency signals, may be eliminated or substantially minimized by utilizing a balanced push-pull circuit of the type shown by Figure 4. This circuit cancels out even harmonies together with the great majority of the intermodulation distortion components so that the noise suppression circuit functions in the manner already described without, however, the presence of distortion produced by intermodulation. In the figure, this circuit is shown as a coupling link having a signal gain of unity so that it may be inserted into existing amplifiers without extensive rearrangement of the ampli fier circuit. The push-pull unit comprises an input transformer 69 having a primary winding BI connected to input terminals 62 and 63, which are shunted by a potentiometer 5G. The transformer 543 has a center tapped secondary winding 65, the tap being grounded by a conductor 86. One terminal of the secondary winding is connected to ground through three series-connected non-linear resistances 6'1, 68, (i9 and the other secondary terminal is grounded through three series-connected non-linear resistances l l, 12, it. A grounded condenser 55 is connected to the junction between resistances Si, 69; a grounded condenser it is connected to the junction between resistances Si, 6!; a grounded condenser ii is connected to the junction between resistances l2, l3; and a grounded condenser 13 is connected to the junction between resistances ii, '52. The junction between resistances 68, 59 is connected by a lead tlii to the control grid of a triode 8i and the junction between resistances i2, 13 is connected by a lead 82 to the control grid of a triode 83. The cathodes of tubes BI, 83 are interconnected by a lead 85 and grounded through a bias resistor 88. The anodes of the tubes are connected, respectively, to opposite terminals of a split secondary winding 81 of an output transformer 88, the center tap of this winding being connected to a positive power supply terminal 89. The secondary winding 90 of transformer 88 is shunted by a potentiometer 91, one end terminal and the arm of this potentiometer being connected, respectively, to output terminals 92 and 93.

This circuit functions in manner similar to that of Figure l in that signal voltage of one phase is applied to the series-connected resistors 5", 553 and 69 while signal voltage of the opposite phase is applied to the resistors ll, 12 and 5'3, the output voltage appearing at the control grids of tubes 8! and 33. Each unit of the circuit consists essentially of a three stage low pass filter, the values of the resistances and, hence, the cutoff frequency being varied by the direct voltage component of the signal. Due to the push-pull circuit arrangement, diiiiculties resulting from interniodulation are eliminated or substantially minimized. The two potentiometers E i and 9! are ganged so that they rotate as a unit. These resistances do not funeuon simply as gain controllers but, rather, potentiometer Gil controls the level at which operation of the noise suppression unit is initiated. That is, this potentiometer controls the level of the incoming signal and, hence, the frequency at which attenuation of the signal begins. The function of the gain control may be performed by a conventional volnine control in a subsequent stage of the ampliiier. it will be understood that suitable impedance matching devices may be added to or substituted for the potentiometers lid, 9i to avoid impedance matching problems with the components of existing amplifiers, when it is desired to add the noise suppression unit to such amplifiers.

in 5, I have disclosed a circuit which has been tested and found to give superior results in operation of a phonograph reproducer. This circuit includes three non-linear resistances 55, 9? and a fourth resistor 93 all connected in series between the cathode of tube ill and a ground tern inal Q3. An impedance 256 is connected between a ground terminal itll and the junction between resistances and it, while a second impedance 4% is connected between a ground terminal E65 and the junction between resistances 8% and 9?. Output terminals tilt and iii! are connected across resistance $1, and resister 98 forms, together with a condenser 5538, an integrating circuit which is connected to an amplifier-rectifier circuit similar to that described by Figure 1 through a lead 39. The present cir cuit is adapted to cut oii unwanted high frequency signals and, accordingly, the impedanccs me, are condensers.

This circuit functions in a similar manner to that of Figure l in that signal voltage is applied to the series connected resistors 25 to 98, inclusive, from the cathode of tube Hl while a direct voltage representative of average signal amplitude is applied to the resistors 96 and till from integrating circuit 98 and 98. The output voltage appears across resistor it will be apparent that the noise suppression unit con sists essentially of two-stage low pass filter, the values of the resistances and, hence, the cut-oil frequency being varied by the direct voltage impressed thereon by the integrating circuit 98 and l In operation, the attenuation of the present circuit is twelve decibels per octave above the cut-oil" frequency rather than six decibels per octave, as in the circuit of Figure 3. This attenuation is ample to eliminate most of the unwanted rughfrequency components, from the output or a. radio or record reproduccr;

While the invention has been described; in con.- nection with a. present, preferred embodiment thereof, it is to be understood that this description is illustrative only and; is, not intended tolimit theinvention, the scope of which is definedby the appended claims.

Having described my invention, I claim;

1. In a noise suppression circuit, a band pass filter comprising a non-linear resistance, a second non-linear resistance connected in series with said first resistance, said resistances having similar voltage-resistance characteristics, and an impedance, for by-passing alternating voltages appearing across one of' saidresistances means for applying a voltage representative of signal amplitude across. both of said resistances, and means for withdrawing. an output appearing across one of said. resistances whereby the pass band. of said filter varies, in accordance with signal amplitude responsive to changesin the ohmic value of said resistances resulting; from changes in signal amplitude.

2. In a, noise suppression circuit, a unit comprising anon-linear resistance and an impedance for by-passing alternating voltages appearing acrosssaicl resistance, a second non-linear resistance connected in series with said first resistance, said resistances having similarvoltage-resistance characteristics, means for applying a voltage representative of signal amplitude across both of said resistances whereby the. voltage drop across the. by-passed resistance is varied in accordance with frequency by said impedance to attenuate signals within a preselected frequency range, and the impressed voltage across said resistances varies the ohmic value thereof to shiftsaid preselected frequency range in accordance with the signal amplitude; and means, for. withdrawing an. output appearing across one of said resistances. 3. In a. noise suppression device,.a circuit comprising a non-linear resistance, and a. unit including an impedance and a second non-linear resistance connected in parallel said second resistance being connected in serieswith said first resistance, said resistances having similar voltage-resistance characteristics, whereby the impedance of said unithas a predeterminediratioto the total impedance of said' circuit. at frequencies where said impedance is large compared to said second resistance, and a substantially smaller ratio at other frequencies. where said-impedance is small compared to. said second resistance, means for applying a voltage across said resistances which is representative of signal amplitude whereby the ohmic value of said resistances varied in accordance with the signal amplitude, thereby to shift the frequency range wherein said impedance is smallicompared. to said shunted resistance in accordance with the signal amplitude, and means for withdrawing an. output appearing across. one of said resistances 4. In a noise. suppression device, a circuit com,- prising. a non-linear resistance and. a unit. including an impedance, and a second non-linear resistance shunted across said. impedance and connected in series with. said: first resistance, said resistances having similar voltagerresistance characteristics, means tor applying, a. signal; volt.- age across said resistances, the, resultant signal voltage appearing across. said resistance impedance unit. being. proportional. to theratio between thetotal. impedance of saidunit. andthe totalimr 10 Dedanceofsaid. circuit, said resultant voltage be ing. substantially attenuated at frequencies Where said impedance is small as compared to said. shunted resistance, and said resultant voltage being substantially unattenuated at other frequenciesmeans for applying a directvoltage across, said resistances which is proportional to the average amplitude. of. the signal, thereby to change the. ohmic. value of said resistances and shift the. frequency band withinv which said resultant voltage is attenuated in accordance. with signal amplitude, and. means for withdrawing an output, appearing across one of said resistances.

In; a noise, suppression device, an electron tube. having. an. anode, a. cathode, and, a control grid, means for supplying operating potentials to the. electrodes at said tube. means for ap lying a, signal; voltage to. said. control grid; av noise suppressi'on circuit. and an integrating circuit. con.- nected in series in the. anode-cathode. ircuit. of said tube, said; noise suppression circuit in ludng a. nonrlinear resist nce nimnedance for. bypassing alternatingvoltages. appearingacross. said resistance and a second non-linear resistance connected in series withsaidfirst resistance, said resistances having, similar voltage-resistance characteristics, said. integratingv circuit including, a resistance and a condenser connected in shunt therewith,.means for. amplifying and rectifying the signal voltage, and means for apply ing. the rectified voltage to said integrating, circuit, thereby to apply: a direct, voltage to) the noise suppression circuit which. is proportional to the average signal; amplitude, whereby the resultant. signal amplitude across said first. rei ance. is, attenuatedby a decrease in. the. react.- ance of. said impedance at all frequencies within a preselected frequency range, and. the impressed direct voltage across said non-linear r e.- sistances varies. the, ohmic value thereof to. shift said preselected frequency range. in. accordance withv the. signal amplitude.

6. In a noise suppression circuit, a. unit. com.- prising a first non-linear resistance, an. imp-ed.- ance for by-passing alternating; voltages. appearing across said first. resistance, a second nonlinear resistance connected in. series. with, said first resistance, said resistances having similar voltage-resistance. characteristics, means; for applying a signal voltage across said series. con,- nected' resistances whereby the, signal amplitude across said resistance-impedance unit. isvaried in accordancewith frequency by said impedance to attenuate signals within a preselected, frequencyrange, the instantaneous. signal voltage across saidseries connected resistances, varying the ohmicv valuethereoi to shift. saidlpreselected frequency range in accordance with the signal amplitude, and means for withdrawing an. output appearing; across one of said resistances.

'7. In a no s suppression device,v three. non..- linearresistances connected in series. and. having similar voltage-resistance characteristics a pair of impedances for by-passing potentials appearing" at the respective junctions between said; re.- sistances, means for applying a. voltage. representative of' signal; amplitude. across. said resist auceswhere y; the voltage drop across oneof; said resistance. un s is. varied, in accordance with requency by said impedances to attenuate sig ials, With n. a. preselected frequency range, and the impressed voltage. across, said resistances varies the ohmicvalue thereof toshiftsaid preselected frequency range accordan with, t signal amplitude. andmeansfor withdrawing an 1 1 output appearing across one of said resistances.

8. In a noise suppression circuit, a low pass filter comprising a pair of non-linear resistances connected in series and having similar voltageresistance characteristics, and a condenser for lay-passing high frequency voltages appearing across one of said resistances, means for applying a voltage representative of signal amplitude across both of said resistances to vary the cut-ofi frequency of said filter in response to resistance variations produced by said voltage in said nonlinear resistances, the cut-on" frequency increasing with increasing signal amplitude and decreasing with decreasing signal amplitude, and means for withdrawing an output appearing across one of said rc isiances.

9. In a noise suppression device, a circuit comprising a non-linear resistance, and a unit including a condenser, and a second nonlinear resistance shunted across said condenser, said second resistance being connected in series with said first resistance, said resistances'having similar voltage-resistance characteristics, means for applying a voltage representative of signal amplitude across said resistances, the resultant voltage appearing across said resistance-condenser unit being proportional to the ratio between the total impedance of said unit and the total impedance of said circuit, said resultant voltage being substantially attenuated by said condenser at frequencies above a preselected. cut -off fre quency at which the reactance of said condenser is equal to the impedance of the resistance shunted thereacross, the ohmic value of said resistances varying in accordance with the average amplitude of the signal whereby the ohmic value of said resistances is decreased as the signal amplitude is increased, thereby to raise the cut-off frequency at which the reactance of said condenser equals the lower impedance of the resistance shunted thereacross, and the ohmic value of said resistances is increased as the signal amplitude is decreased, thereby to lower said outofi Irequency, and means for withdrawing an output appearing across one of said resistances.

10. In a noise suppression device, a circuit comprising a non-linear resistance and a unit including a condenser, and a second. non-linear resistance shunted across said condenser and connected in series with said first resistance, said resistances having similar voltage-resistance characteristics, means for applying a signal voltage across said resistances, the resultant signal voltage appearing across said resistance-condenser unit being proportional to the ratio between the total impedance of said unit and the total impedance of said circuit, said resultant voltage being substantially attenuated by said condenser at frequencies above a preselected cutoff frequency at which the reactance of said condenser is equal to the impedance of the resistance shunted thereacross, means for applying a direct voltage across said resistances which is proportional to the average amplitude of the signal whereby the ohmic value of said resistances is decreased as the signal amplitude is increased, thereby to raise the cut-off frequency at which the impedance of said condenser is equal to the lower impedance of the resistance shunted thereacross, and the ohmic value of said resistances is increased as the signal amplitude is; decreased, thereby to lower the cut-off frequency, and means for withdrawing an output appearing across one of said resistances.

11. In a noise suppression circuit, an electron tube having an anode, a cathode, and a control grid, means for supplying operating potentials to the electrodes of said tube, means for applying a signal voltage to said control grid, a noise suppression circuit and an integrating circuit connected in series in the anode-cathode circuit of said tube, said noise suppression circuit including a non-linear resistance, a condenser for by-passing high frequency voltages appearing across said resistance, a second non-linear resistance connected in series with said first resistance, said resistances having similar voltage-resistance characteristics, said integrating circuit including a resistance and a condenser connected in shunt therewith, means for amplifying and rectifying the signal voltage, and means for applying the rectified voltage to said integrating circuit, thereby to apply a direct voltage to the noise suppression circuit which is proportional to the average signal amplitude, whereby the signal amplitude across said first resistance is attenuated by said condenser at all frequencies above a preselected cut-off frequency, and the impressed direct voltage across said non-linear resistances raises the cut-off frequency as the signal amplitude increases and lowers the cut-oi? frequency as the signal amplitude decreases.

12. In a noise suppression device, an electron tube having an anode, a cathode, and a control grid, means for supplying operating potentials to the electrodes of said tube, means for applying a signal voltage to said control grid, a noise suppression circuit and an integrating circuit connected in series in the anode-cathode circuit of said tube, said noise suppression circuit including three non-linear resistances connected in series and having similar voltage-resistance character istics, and a pair of condensers connected to the respective junctions between said resistances to by-pass signal voltages appearing at said junctions, said integrating circuit including a resistance and a condenser connected in shunt therewith, means for amplifying and rectifying the signal voltage, and means for applying the rectified voltage to said integrating circuit, thereby to apply a direct voltage to the noise suppression circuit which is proportional to the average signal amplitude, whereby the signal amplitude across one of said resistances is attenuated by said condensers at all frequencies above a preselected cut-off frequency, and the impressed direct voltage across said non-linear resistance raises the cut-ofi frequency responsive to increases in signal amplitude and lowers the cut-off frequency responsive to decreases in signal amplitude.

13. In a noise suppression circuit, a high pass filter comprising a pair of non-linear resistances connected in series and having similar voltageresistance characteristics, an inductance for bypassing low frequency signals appearing across one of said resistances, and means for applying a voltage representative of signal amplitude across said resistances whereby the ohmic value of said resistances varies inversely with signal amplitude to change the cut-ofi frequency of said filter, and means for withdrawing an output appearing across one of said resistances.

14. In a noise suppression circuit, a unit comprising a non-linear resistance and an inductance for my-passing alternating voltages appearing across said resistance, a second non-linear resistance connected in series with said first resist ance, said resistances having similar voltageresistance characteristics, means for applying a 13 voltage. representative of, signal amplitude across said resistances whereby the voltage drop across said resistance is varied by: said inductance to attenuate signals having a frequency lower than apreselected: cut-off frequency, and the impressed voltage across said resistances varies the ohmic value thereof to lower the cut-off frequency as the signal level increases;v and to raise the cutroff frequency as the signal level decreases, and, means forwithdrawing an output, appearing across one of said resistances.

15. In a noise suppression circuit, an electron tube having an anode, a cathode, and a control grid, means for supplying operating potentials to the electrodes of said tube, means for applying a signal voltage to said control grid, a noise suppression circuit and an integrating circuit connected in series in the anode-cathode circuit of said tube, said noise suppression circuit including a non-linear resistance and an inductance for by-passing low frequency voltages appearing across said resistance, and a second non-linear resistance connected in series with said first reslstance, said resistances having similar voltageresistance characteristics and equal ohmic values, said integrating circuit including a resistance and a condenser connected in shunt therewith, means for amplifying and rectifying the signal voltage, and means for applying the rectified voltage to said integrating circuit, thereby to apply a direct voltage to the noise suppression circuit which is proportional to the average signal amplitude, whereby the signal amplitude across said first resistance is attenuated by said inductance at all frequencies below a preselected cut-off frequency, and the impressed direct voltages across said non-linear resistances lowers the cut-off frequency as the signal amplitude increases and raises the cut-off frequency as the signal amplitude decreases.

16. In a noise suppression circuit, an electron tube having an anode, a cathode, and a control grid, means for supplying operating potentials to the electrodes of said tube, means for applying a signal voltage to said control grid, a noise suppression circuit and an integrating circuit connected in series in the anode-cathode circuit of said tube, said noise suppression circuit including three non-linear resistances connected in series and having similar voltage-resistance characteristics, a pair of by-pass inductances connected to the respective junctions between said resistances to by-pass a portion of the signal voltages appearing at said junction, said integrating circuit including a resistance and a condenser connected in shunt therewith, means for amplifying and rectifying the signal voltage, and means for applying the rectified voltage to said integrating circuit, thereby to apply a direct voltage to the noise suppression circuit which is proportional to the average signal amplitude, whereby the signal amplitude across one of said resistances is attenuated by said inductances at all frequencies below a preselected cut-oil frequency, and the impressed direct voltage across said nonlinear resistances lowers the cut-off frequency as the signal amplitude increases and raises the cut-off frequency as the signal amplitude decreases.

1'7. In a noise suppression circuit, two units connected in push-pull circuit arrangement, each of said units being constructed in accordance with claim 6, means for applying a signal voltage of one phase across one of said units, means for applying a signal voltage of opposite phase across the. other unit; and means for taking: off, output voltage of one, phase. from.- one; of; the. resistances: in one unitand for taking off output; voltage 0.1"? opposite phase from one. of the resistances oi the other unit.

18; The combination in accordance with claim, fi wherein: said impedance comprises a. condenser whereby the signal amplitude across said resistance-condenser unit is attenuated by: said condenser at all frequencies above a preselected cut-off frequency and the impressed voltage: across said resistances: varies the ohmic. value thereof to raise the cut-oiffrequency as the signal level increases and to lower the cut-off frequency as the signal. level decreases.

19. The combination in accordance with claim 6 wherein said impedance comprises a condenser whereby the signal amplitude across said resistance-condenser unit is attenuated by said condenser at frequencies above a preselected cut-off frequency where the reactance of said condenser is equal to the impedance of the resistance shunted thereacross, and the instantaneous signals voltage across said resistances increases the ohmic value thereof as the signal amplitude decreases, thereby to lower the cut-off frequency at which the reactance of said condenser equals the higher impedance of said shunted resistance, and said instantaneous signal voltage decreases the ohmic value of said resistances as the signal amplitude increases, thereby to raise the cut-off frequency.

20. The combination in accordance with claim 7 wherein each of said impedances comprises a condenser whereby the signal amplitude across one of said resistances is attenuated by said condenser at frequencies above a preselected cutoff frequency where the reactance of said condenser bears a predetermined relation to the impedance of said resistances, the voltage across said resistances varying in accordance with the amplitude of said signal to lower the cut-off frequency responsive to a decrease in signal amplitude and to raise the cut-off frequency responsive to an increase in signal amplitude.

21. In a noise suppression circuit, two units each constructed in accordance with claim 20, means for applying a signal voltage of one phase across one of said units, means for applying a signal voltage of opposite phase across the other of said units, a pair of electron tubes connected in push-pull circuit arrangement, each having an anode, a cathode, and a control grid, means for applying operating and bias potentials to the electrodes of said tubes, means connecting a junction between two resistances of one unit to one of said control grids, means connecting a junction between two resistances of the other unit to the other of said control grids, an output transformer, and means connecting the anodes of said electron tubes to said output transformer.

22. In a noise suppression circuit, two units connected push-pull circuit arrangement, each of said units being constructed in accordance with claim 20, means for applying a signal voltage of one phase across one of. said units, means for applying a signal voltage of opposite phase across the other unit, and means for taking off an output voltage of one phase from one of the resistances in one unit and for taking off out-put voltage of opposite phase from one of the resistances of the other unit.

23. The combination in accordance with claim 6 wherein said impedance comprises an inductance whereby the signal amplitude across said resistance-inductance unit is attenuated by said inductance at frequencies above a preselected cutofi frequency where the reactance of said inductance is equal to the impedance of the resistance shunted thereacross, and the instantaneous signal voltage across said resistances increases the ohmic value thereof as the signal amplitude decreases, thereby to lower the cut-off frequency at which the reactance of said inductance equals the higher impedance of said shunted resistance, 10

References Cited in the file of this patent Number UNITED STATES PATENTS Name Date Ford June '15, 1937 Bartels Dec. 7, 1937 Chesnut Jan. 10, 1939 Rechnitzer July 25, 1939 Cousins Jan. 4, 1944 Andrews Sept. 28, 1948 

